Refrigeration systems, particularly refrigeration systems in mobile or locomotive applications, are highly restricted in terms of the space available to them. Nevertheless, buyers of such systems demand high performance, and they particularly demand this performance under the most trying conditions. An example may be an automobile air-conditioning system on a hot day in slow traffic. There may be only a small temperature difference between the heat rejected and the sink into which the heat is rejected. The demand on the system, however, or the quantity of heat rejected, may be very great if the automobile has several passengers. In slow traffic with a small amount of ram air, the cooling air heat exchange medium is at a triple disadvantage: the air itself will be at a higher temperature; at slow speeds, the air volume impinging on the heat exchanger will be minimal; and less air mass is available because air is less dense at higher temperatures.
Other examples of mobile applications may include refrigeration systems for truck cabs, over-the-highway refrigerated trailers, refrigerated railcars, passenger trains, and aircraft passenger sections. While these examples suggest locomotive or mobile applications, space may also be at a premium in stationary applications, such as any refrigeration system. These may include, but are not limited to, building air-conditioning systems, smaller air-conditioning or chilling systems, process chillers such as those used on machine tools, refrigeration equipment, compressors, and in short, any application that requires heat transfer. Space is ever at a premium for mechanical equipment or systems, and any heat exchanger or condenser that can be made smaller or more efficient is welcome.
Focusing on the automotive applications, and particularly on the refrigeration system used for air-conditioning, engineers have found that extra space under the hood is very scarce. There is an additional problem, in that space is not the only consideration, but low cost and low weight is also necessary. Any air-conditioning or refrigeration system used in millions of automobiles must be economical. Therefore, many heat exchangers or radiators used in automotive applications tend to have cross-flow arrangements, that is, the coolant tends to flow from left to right, rather than up and down. Cross-flow under the hood allows a longer flow path, creating more surface area for heat exchange, and allowing for a smaller number of tubes in a typical air-cooled radiator.
There are efficiency problems in using a cross-flow heat exchanger in these applications. The most obvious problem may arise in considering the physical changes to the refrigerant in the heat exchange process. In a typical refrigeration system, the condenser receives gaseous refrigerant which has picked up heat that is absorbed from the cooled area or system and compressor. Refrigerants are cooled into a liquid state when they pass through the condenser. However, once the refrigerant or coolant has condensed, it will reside in the bottom half of a heat exchange channel or tube into which it was introduced. Liquid coolant in the bottom of a tube or channel will provide a barrier to the heat path: the heat must now travel from the gaseous refrigerant, through the liquid at the bottom of the tube or channel, and only then through the thickness of the tube or channel, before it can be rejected into cooling air, ram air, or other heat rejection medium.
Even if the heat exchanger uses a multi-pass flow, each pass will see some condensation, and the efficiency of each pass will be degraded at least to the extent and depth of the liquid condensate. What is needed is a heat exchanger that is not “fouled” by liquid condensate. What is needed is a condenser that does not permit such a barrier to accumulate and block heat flow. What is needed is a condenser that quickly and efficiently separates gaseous refrigerant from its condensed liquid, allowing for better efficiency in the condenser and higher heat exchange capacity for the refrigeration system of which it is a part.